Waste disposal of contaminated drill cuttings from geological drilling using drilling fluid systems containing mineral oil

ABSTRACT

Described is the use of ecologically compatible, and more particularly of biologically degradable, oils having flash points of above 80° C. for washing mineral oil-loaded drill cuttings prior to the disposal thereof by off-shore and/or on-shore deposition. For the use as wash oils there are preferred aerobically and/or anaerobically degradable low-viscosity oils are employed which are also suitable for being used as a mixture component of the continuous oil phase of W/O invert drilling fluids.

This application is a continuation of application Ser. No. 08/256,495filed on Oct. 14, 1994 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Oil-based drilling fluids and W/O invert drilling muds based thereon areof outstanding importance in the modern technology of geologicaldevelopment, for example for the exploitation of crude oil and/ornatural gas deposits. The use of the new drilling fluid systems is ofparticular importance in the marine sector, but it is not limitedthereto. The technology of drilling by the use of oil-based W/O invertsystems finds general use also in terrestrial drilling, for example ingeothermal drilling, water drilling, in carrying out geo-scientificdrilling and in drilling in the field of mining.

Liquid sweeping systems of the type concerned here which are by farsuperior to the water-based systems, more particularly, in the sectorsof off-shore drilling or in the penetration of water-sensitive layers,are generally used as so-called invert emulsion muds which consist of athree-phase system: Oil, water and finely divided solids. They arepreparations of the type of W/O emulsions, i.e. the aqueous phase hasbeen heterogeneously distributed in a finely dispersed state in thecontinuous oil phase. For stabilizing the system altogether and forestablishing the desired performance properties, a multiplicity ofadditives is provided, more specifically emulsifiers or emulsifiersystems, weighting agents, fluid-loss additives, alkali reserves,viscosity modifiers and the like. As to details, reference is made, forexample, to the publications P. A. Boyd et al., "New Base Oil Used inLow-Toxicity Oil Muds", Journal of Petroleum Technology 1985, 137 to142, and R. B. Bennett, "New Drilling Fluid Technology-Mineral Oil Mud",Journal of Petroleum Technology 1984, 975 to 981, as well as theliterature quoted therein.

Oil-based drilling fluids were initially composed of diesel oilfractions containing aromatic constituents. For the purposes ofdetoxification and reducing the ecological problems thus created it wasthen proposed to use hydrocarbon fractions largely free from aromaticcompounds--today also denoted as "non-polluting oils"--as the continuousoil phase; hereto cf. the literatur cited above. Nevertheless, even anelimination of the aromatic compounds from the mineral oils only leadsto a restricted reduction of the environmental problems caused by thedrilling fluids of the above type. Here, particular difficulties arisefor the waste disposal of the so-called "cuttings" removed from thesweeping fluid phase, i.e. the rock cuttings formed upon drilling andconveyed upwardly along with the drilling fluid. In practice, theseamounts of rock cuttings produced are separated by one or more step(s)of sieving and/or by further separating steps such as centrifugationfrom the major amount of the recycled drilling fluid phase. Theresulting removed drill cuttings have been wetted with considerableamounts of the oil-based drilling fluid phase. If drilling fluid systemsbased on non-degradable mineral oils as the continuous oil phase areused, any disposal of drill cutting waste by dumping it down to thebottom of the ocean is now out of consideration. Ecologicalinvestigations have revealed that contaminations by mineral oils arestill present even a long time after they were deposited and still canadversely affect the particularly sensitive marine eco-system. The sameproblems will arise upon work with water-based O/W emulsion fluids.

2. Discussion of Related Art

The relevant technolgy has for some time recognized the significance ofa use of ecologically compatible and especially of biodegradable oilphases in connection with the composition of oil-containing drillingfluid systems. Thus, the U.S. Pat. Nos. 4,374,737 and 4,481,121 describeoil-based drilling fluids wherein non-polluting oils are reported tohave been used. As the non-polluting oils there have been mentioned,besides mineral oil fractions which are free from aromatics, vegetableoil of the peanut oil, soybean oil, linseed oil, corn oil and rice oiltypes and also oils of animal origin such as whale oil. Subsequent moredetailed investigations have shown that the use as contemplated in priorart of readily degradable oils of vegetable and/or animal origin cannotbe realized for practical reasons. The rheological properties of suchoil phases can not be controlled over the temperature range as requiredin practice of from 0° C. to 5° C., on the one hand, up to 250° C. andhigher, on the other hand.

Applicants, in a greater number of published, and in part stillunpublished, protective rights describe proposals for replacing themineral oil fractions by ecologically compatible readily degradable oilphases. Among these, four different types of substitute oils have beenpresented which may also be used as mixtures. They include selectedoleophilic monocarboxylic acid esters and corresponding polycarboxylicacid esters, at least largely water-insoluble alcohols which are fluidunder the operation conditions, corresponding ethers and selectedcarbonic acid esters. In summary, reference is made here to thepublished older applications DE-A-38 42 659, DE-A-38 42 703, DE-A-39 07391, DE-A-39 07 392, DE-A-39 03 785, DE-A-39 03 784, DE-A-39 11 238,DE-A-39 11 299, DE-A-40 19 266 and DE-A-40 18 228. All of the printedpublications mentioned here relate to the field of oil-based drillingfluid systems, especially of the W/O inverted type.

Today, oil-based drilling fluid systems based on the appropriatemonocarboxylic acid esters and/or based on ethers of monofunctionalalcohols have shown their value in practical use and have proven theirmanifold superiority over the mineral oil-based drilling fluids,especially of the W/O invert type. One of the important facilitationsprovided by working with ecologically compatible and, more specifically,bio-degradable oils of this kind is that it simplifies the disposal ofthe drill cuttings wetted with the oil fluid. If, for example, the rockcuttings separated by sieving and/or centrifugation are deposited on thebottom of the ocean, then the dumping area will not become polluted. Themarine ecosystem remains substantially undisturbed; for example, theintroduced ester-based drilling fluid systems will virtually have beendecomposed after comparably short periods of time.

The teaching of the invention proceeds from the object, on the ground ofthe knowledge as available today on the ecological compatibility of thebiologically degradable oil phases in systems of the type concernedhere, to open up the possibility in practical operation to employdrilling fluid systems containing mineral oil, and especially W/O invertfluid muds predominantly based on mineral oils or mineral oil fractions,respectively, without having to compromise the advantages as obtained inparticular for the deposition of the drill cuttings with thebio-compatible systems described above. The use of mineral oils or ofmineral oil fractions, respectively, as the main component of the invertfluid systems - but also in water-based fluids of the O/W type--may bedesirable, for example under the aspects of availability and/or aspectsof costs, if at the same time it can be reliably guaranteed thatundesirable contaminations of the environment, and more particularly ofthe marine eco-system, by these per se ecologically toxic sweepingliquids are avoided. It is general practice up to date, to transport themineral oil-contaminated cuttings to land and there to clean--forexample by washing them with aqueous surfactant solutions--and/or todeposit them.

The teaching of the invention proceeds from the concept of that theobject aimed at is to be attained via a route that has not beendescribed hitherto: The invention to solve the problems outlined aboveby an exchange of the oil phase on the rock cuttings removed from thedrilling fluid. The incompatible mineral oil-based oil phase wetting therock cuttings is to be replaced especially by an oil phase based onecologically compatible, and more particularly biologically degradable,oils. However, this additional measure is to be rendered possible at aneconomically reasonable and tolerable expenditure of material and money.

3. Description of the Invention

Accordingly, the invention relates to the use of ecologicallycompatible, and more particularly of biologically degradable, oilshaving flash points of above 80° C. for washing mineral oil-loaded drillcuttings prior to a disposal thereof by off-shore and/or on-shoredeposition. As the ecologically compatible wash oils according to theinvention there are used, more specifically, aerobically and/oranaerobically degradable low-viscosity oils which themselves aresuitable due to their product properties--for example rheology,volatility, toxicology, miscibility and the like--for being used indrilling fluids and, more specifically so, as a mixture component of thecontinuous oil phase of W/O invert drilling fluids.

Particularly suitable ecologically compatible and especiallybiologically degradable wash oils fall within the substance classes asinitially quoted of oleophilic monocarboxylic acid esters and oleophilicpolycarboxylic acid esters, corresponding ethers or alcohols and/or thepartial ethers thereof, as well as oleophilic carbonic acid esters, suchas those described in detail in Applicants' above-mentioned olderapplications. These aerobically and/or anaerobically degradable oils aspreferred according to the invention are also designated hereinbelow as"oxygen-functionalized wash oils" or as "O-functionalized wash oils".

DETAILED DESCRIPTION OF THE INVENTION

The teaching of the invention proceeds from the following concept:

The amounts of drill cuttings, which under the conditions of operationin practice are to be continuously removed from the oil-containingdrilling mud loaded with drilling cuttings contain considerable amountsof oil phase on their surfaces, if a W/O invert fluid is employed as theoil-based drilling fluid. In the case contemplated by the invention,said drilling fluid contains substantial amounts--mostly at leastpredominant amounts--of mineral oil. The drilling fluid, after thecuttings have been removed therefrom, is recirculated into the borehole. This circulating stream of the mineral-oil based drilling fluidmay be prevented from coming into undesirable contacts with thesensitive eco-system.

According to the invention, the waste disposal--which so far has beenproblematic--of the mineral oil-contaminated cuttings is now effected bywashing same with O-functionalized wash oils. Hereby the ecologicallyincompatible mineral oils is displaced from the cuttings, whereby theoil load on the cuttings is replaced by the biologically degradableO-functionalized wash oil. In this form, the cuttings now wetted withaerobically and/or anaerobically degradable oils may now withoutscruples be deposited off-shore and/or on-shore. Thus, the facilitationaimed at in the removal of the drill cuttings has been ensured.

The economic efficiency of such an operational step--illustrated by wayof the case of the particularly important W/O invert-based fluidsystems--is optimized by the following preferred elements of actingaccording to the invention: The amount of drill cuttings upwardlyconveyed with the circulating stream of the W/O drilling fluid is knownto be relatively small, relative to the circulated amount of the W/Oinvert mud. It usually amounts to at best a few percent by volume. Thus,comparably low are the amounts of O-functionalized wash oils requiredfor washing said oil-loaded cuttings; in these considerations it isfurther to be noted that the progress in drilling achieved in the courseof the operation makes it necessary to continuously replenish thecirculating stream with further additional quantities of W/O invert mud.

In one preferred embodiment of the invention there are employed washoils which do not only satisfy the requirement of biologicalcompatibility and degradability in the respective eco-system. Thepreferred wash oils according to the invention have been selectedadditionally under the aspect of that they themselves are capable oftaking over the function of the continuous oil phase in this drillingfluid, at least as a mixture component in admixture with the mineraloils of the W/O invert mud. The advantage of acting within the meaningof this provision according to the invention will be immediatelyapparent: The used-up wash oils contaminated with mineral oil are quitesimply disposed of by feeding them into the circulating stream of theW/O invert mud and, hence, making them a component of the drilling fluiditself. Thus, the teaching according to the invention allows thebiologically compatible and degradable O-functionalized wash oils to beutilized to an optimum degree in a comparably low amount, while theessentially mineral oil-based drilling fluid systems are maintainedwithout any need to compromise the crucial advantage of the facilitateddisposal of the drill cuttings. Comparable considerations are applicableto the operation using water-based emulsified mineral oil-containing O/Wemulsion fluids.

The selection of particularly suitable biologically degradable wash oilsis governed by the double functionality demanded therefor within thescope of the invention. The wash oils, on the one hand, as preferredlylow-viscosity organic liquids, are desired to be readily miscible withthe mineral oil phase, e.g. the W/O invert mud, in the course of thewashing operation, so that an efficient scouring of the cuttings withlimited amounts of the O-functionalized wash oil will become possible.On the other hand, these wash oils will have to be efficient componentsof the continuous oil phase of the invert mud, once they will have beenadmixed to the drilling fluid. Thus, preferred O-functionalized washoils exhibit pour points and setting points of not above 5°0 C., andespecially of below 0° C. Particularly suitable may be wash oils whichhave pour points and setting points of -5° C., and especially of below-15° C. Biodegradable oils suitable in practice often possesssolidification values of below -25° C. to -30° C. The viscosity of thewash oils may be important especially in the first step of the usethereof, i.e. in the washing operation of the cuttings loaded with,e.g., invert mud. Here it may be convenient for developing the optimumsolvent and wash oil qualities, to employ liquid phases which arereadily mobile and also in lower temperature ranges have a sufficientlylow viscosity. There is usually applicable that the O-functionalizedwash oils should be oils that are flowable and pumpable in thetemperature range of from 0° C. to 5° C. and which possess preferredBrookfield (RVT) viscosities of below 80 mPa.s, and preferably of below60 mPa.s. Particularly suitable wash oils may exhibit viscosities of upto about 45 mPa.s in the temperature range as indicated.

In this connection, however, another additional preferred workingelement of the invention is to be taken into consideration, as follows:In an important embodiment of the invention, the washing procedure,still to be described in detail hereinbelow, of the mineral oil-loadedcuttings can be carried out at elevated temperatures, especially in thetemperature range up to 100° C. and preferably within the temperaturerange of from about 35° C. to 80° C. The drilling fluid loaded withcuttings will already reach elevated temperatures at a limited depth ofthe bore hole, so that in the course of practical operation thedischarged fluid and, thus, also the cuttings contained therein, is at atemperature level of from 35° C. to 60° C. In one preferred embodiment,the teaching according to the invention makes particular use thereof inwashing the cuttings by that the O-functionalized wash oil is alsoheated at comparable or even higher temperatures for subsequent use inthe washing procedure. This in a per se known manner involves areduction in the viscosity of the wash oil and, thereby, the improvementof its function demanded here as washing agent or solvent, respectively.

O-Functionalized Washing Oils Preferred According to the Invention

In summary, general reference may once more be made here to Applicants'previously published--and in part still unpublished--older applicationsas throughout relating to the field of oil-base drilling fluid muds, andespecially those of the W/O invert type. From these applications thereare in detail derivable suitable esters of monocarboxylic acids,suitable esters with a concomitant use of polycarboxylic acids, suitableethers as well as suitable oleophilic alcohols including the partialethers thereof and suitable oleophilic carbonic acid esters. All of thesubject matters of these mentioned older applications relating to thecertain types of the respectively disclosed O-functionalized oil phasesare incorporated herein by reference.

Especially important representatives of the wash oils within the scopeof the invention are specified within the classes of oleophilic estersof carboxylic acids and of oleophilc ethers. More particularly,oleophilic carboxylic acid esters may be especially suitablerepresentatives, due to their ability to undergo an aerobic biologicaldegradation as well as an anaerobic biological degradation. Today,so-called sea-floor tests are available to the art which tests furnishevidence of the safety of the disposal of ester-loaded cuttingsresulting from drilling fluids of the kind as here concerned.

There is consistently applicable to the oil phases and/or oil mixedphases as to be preferably used according to the invention that flashpoints of at least about 100° C., and preferably flash points aboveabout 135° C. are required for reasons of a safe operation. Values thatare distinctly higher, especially those in excess of 150° C., may beparticularly expedient.

There is further consistently applicable to those various oil phasesusable within the scope of the invention that are potentiallysusceptible to hydrolysis, not only that the wash oils themselves, i.e.the respective selected ester oil or ester oil mixture, must beecologically compatible, but also that no toxicological and especiallyno inhalation-toxicological danger will be induced upon a partialsaponification in practical use. Within the scope of the mentioned olderapplications there has been described in great detail that here, moreparticularly, the various representatives of ester oils are referred to,with the monofunctional alcohols from the esters formed being again ofparticular significance here. In comparison to polyfunctional alcohols,the lower members of the monofunctional alcohols are highly volatile, sothat here a partial hydrolysis may cause exposure to secondary danger.Accordingly, in the classes of the various ester oils thosemonofunctional alcohols included in the use, or the moieties of suchalcohols, have been chosen so that they have at least 6 carbon atoms,and preferably at least 8 carbon atoms, in the molecule thereof.Hydrolysis-resistant ethers may be particularly important for thepractical use as components of the continuous phase of the drill holetreating agents--subsequently to the use thereof as washing and/orcleaning agents for the cuttings to be stripped of mineral oilcontaminations.

As ester oils which are ecologically compatible and biologicallydegradable, there have proven to be useful, more specifcally, esters ofmonocarboxylic acids which then, in a preferred embodiment of theinvention, are derived from at least one of the following subclasses:

a) Esters of C₁₋₅ -monocarboxylic acids and mono- and/or polyfunctionalalcohols, whereof the moieties of monohydric alcohols comprise at least6 carbon atoms and preferably at least 8 carbon atoms and the polyhydricalcohols preferably have from 2 to 6 carbon atoms in the molecule,

b) Esters of monocarboxylic acids of synthetic and/or natural origincomprising from 6 to 16 carbon atoms, and more specifically esters ofaliphatic saturated monocarboxylic acids and mono- and/or polyfunctionalalcohols of the kind mentioned in a),

c) Esters of olefinically mono- and/or polyunsaturated monocarboxylicacids having at least 16, and especially 16 to 24 carbon atoms andespecially monofunctional straight-chain and/or branched alcohols.

Starting materials for recovering numerous monocarboxylic acids fallingunder these subclasses, especially those having a higher number ofcarbon atoms, are vegetable and/or animal fats and oils. There may bementioned tallow, coconut oil, palm kernel oil and/or babassu oil,especially as feedstock for the recovery of monocarboxylic acids of theprevailing range up to C₁₈ and of essentially saturated components.Ester oils of vegetable origin based on olefinically mono- andoptionally poly-unsaturated carboxylic acids of the range of C₁₆₋₂₄ are,for example, palm kernel oil, peanut oil, castor oil, sunflower oil, andespecially rapeseed oil. But also tall oil-based esters fall within thiscategory of comparably highly mobile monocarboxylic acid esterscomprising olefinically mono- and/or polyunsaturated moieties. Inaddition to such esters largely derived from natural material,components synthetically recovered are also important structuralelements for ecologically compatible and bio-degradable oil phases onthe side of the carboxylic acids as well as on the side of the alcohols.

Particularly important monocarboxylic acid esters for the use accordingto the invention have been described in the older applications DE-A-3842 659 and DE-A-38 42 703, both of which are incorporated here byreference. The olefinically unsaturated esters described in thefirst-mentioned one of these two printed publications, with respect tothe composition thereof, are members of the above-mentioned subclass c),while the especially important esters according to the DE-A-38 42 703belong to the above-mentioned subclass b) referring to suitablemonoesters. Here of interest are in the first place esters that areflowable and pumpable within the temperature range of from 0° C. to 5°C.--of monofunctional alcohols having up to 12 carbon atoms andaliphatically saturated monocarboxylic acids having from 12 to 16 carbonatoms or mixtures thereof with minor quantities of other monocarboxylicacids. Ester oils of this kind which contain at least about60%--relative to the mixture of carboxylic acids--of esters derived fromaliphatic C₁₂₋₁₄ -monocarboxylic acids may be especially suitable. Animportant representative of this class of ester oils is sold by one ofApplicants under the trade name of "PETROFREE" (registered trademark).

Ester-based wash oils are always suitable, if a sufficient stability ofthe esters in the W/O invert muds can be expected under the conditionsof use in practice. With a view to details, reference is made to thecited printed publications relating to the use of such esters indrilling fluids. If, under the operation conditions of the drillingfluids, high requirements are set for the stability to hydrolysis--forexample in operating at high temperatures, especially in the temperaturerange from 150° C. upward--, then the use of ether oils asO-functionalized wash oil may be appropriate. A subsequent admixture tothe mineral oil-based drilling fluid of these ether oils does not leadto any substantial restriction of the temperature resistance of such aninvert fluid.

Among the wash oils of the type here concerned, preferred wash oils areethers, mixed ethers and ether mixtures of monofunctional alcoholshaving at least 6 carbon atoms, which wash oils may also contain mixedethers with incorporated moieties of polyhydric alcohols. Within thescope of the ether oils as contemplated here, particularly suitable arethose compounds that are derived at least in part, and preferably atleast predominantly, from straight-chain monofunctional alcohols, theethers of the respective monoalcohols having from 8 to 12 carbon atomsand/or the mixed ethers thereof with di- to tetrahydric alcohols havingstraight-chain or branched carbon chains being particularly preferred.

In connection with the use of the ester oils and/or ether oils asparticularly featured here, one significant additional advantage may beindicated here which, as the result of acting according to theinvention, will become noticeable in the mineral oil-based drillingfluid--the W/O invert emulsion, but also in O/W emulsions--: Mineraloil-containing drilling fluids, in comparison to merely water-baseddrilling fluids, are known to possess clearly improved lubricatingproperties; nevertheless, the improvement in lubricity attainable withthe mineral oil is limited. The use in practice of, for example, theester-based drilling fluids based on the commercial product "PETROFREE"(registered mark) as mentioned has shown that substantial improvementsin the lubrication in practical operation are attainable via the esteroil phase. Within the scope of the invention, the O-functionalized washoils are ultimately added to the W/O invert fluid. Here they will becomea constituent of the continuous oil phase which is mineral-based to atleast a substantial proportion. Due to admixing the O-functionalizedwash oils to said drilling fluid, the lubricating effect providedthereby is substantially improved, so that here also a significantsecondary improvement may be utilized. This will matter not only for theregular operation of vertical drilling, but this improvement in thelubricity will gain further practical importance especially in inclineddrilling, derived drilling and/or horizontal drilling.

It may be desirable, under the aspect of said improvements of animportant product quality, as an O-functionalized wash oil to employsuch a component of said type which, in admixture with the W/O invertmain fluid, enables an optimization of the lubricating effect to beachieved. Here, wash oils comprising a plurality of ester groups, forexample lower-viscosity polycarboxylic acid esters according to DE-A-4019 266 and/or carbonic acid diesters according to DE-A-40 18 228 may bethe preferred working agents.

It is only for the sake of completeness that it may be mentioned thatany optional admixtures of the O-functionalized wash oils of the kinddescribed in the cited older applications may be employed within thescope of the teaching according to the invention.

Further Details of the Teaching According to the Invention

Washing the drill cuttings contaminated with the mineral oil-baseddrilling fluid by using the biologically degradable O-functionalizedwash oils may be performed batchwise and/or continuously in any optionalmanner. The process may be operated in one step or in more than onestep. The washing procedure may make use of cocurrent streams, but alsoof counter-current streams of the material to be washed and the washfluid.

In a preferred embodiment, washing may be carried out immediately afterthe recovery of the mineral oil-contaminated cuttings--i.e. in off-shoredrilling, for example, on the platform--while, of course, the cuttingsto be decontaminated may also be conveyed to a washing plant installedat some other place. The technologies employed in the washing processare in accordance with the knowledge from prior art, for example fromcleaning oil-loaded and soiled earth or rock. Within the scope of thecleaning procedure according to the invention, it is advantageous that,as the wash oil, there is employed a non-aqueous hydrophobic oil phase,while not any aqueous surfactant solutions as have been proposedhitherto for washing the cuttings. The cuttings, more specifically thecuttings from off-shore drilling, are known to frequently be swellableclays which tend to undesirable secondary reactions upon a use ofaqueous washing solutions.

Within a multistep process of washing cuttings, one particularembodiment of the invention deserves to be highlighted: This embodimentcomprises at least two washing steps. In a first washing step, thecuttings to be cleaned are washed with a mineral oil-based oil phase.Here, more specifically, the hydrocarbon fraction may be used thatconforms to the continuous oil phase from the mineral oil-based W/Oinvert emulsion. In this first washing step, an appropriately thin-fluidmineral oil in a particularly simple manner may successfully scour offthe W/O invert mud remainders from the surface of the cuttings, so thatin the subsequent washing step(s) only an exchange of the oil phaseswill have to be effected, i.e. the replacement of mineral oil byO-functionalized wash oils as taught by the invention. Also here, a fulldisposal of the respective wash oils is feasible in the described mannerin that said wash oils are admixed to the W/O invert drilling where theywill become a constituent of the continuous oil phase.

As has already been mentioned, it may be convenient to employ anelevated working temperature of up to 90° C., for example one within therange of from about 45° C. to 75° C., in the washing step(s), in orderto render washing easier or allow a shorter washing period.

A further facilitation of work for the oil washing is provided by thefollowing embodiment of the working step(s): Washing may be effected inthe presence of elevated concentrations of emulsifiers, and especiallyemulsifiers of the W/O type. In this case it is convenient to admix theadditional amount of emulsifier to the wash oil. This may considerablyfacilitate not only the absorption in the wash oil of the amounts of oilto be removed from the surface of the cuttings, but also the removal ofthe amounts of solids adhered thereto--weighting agents, fluid-lossadditives and the like. In one embodiment, as the emulsifier additivesthere are conveniently chosen those W/O emulsifiers that are used in thedrilling fluid for making the W/O invert mud. In practice suitableemulsifiers are, for example, nitrogen-containing compounds from theclasses of polyamines and/or polyamidoamines. In this context, referencemay be made to the pertinent literature of the art, for example G. R.Gray et al., "Composition and Properties of Oil Well Drilling Fluids",4th Edition, Gulf Publishing Cp., Houston, London 1981, thereinespecially sub-chapter 7 and the literature cited therein. Oneparticularly suitable type of emulsifier is the product sold by oneApplicant under the tradename of "EZ-mul". Emulsifiers of the kindconcerned here are commercially traded as highly concentratedpreparations of the active ingredient and may be admixed to the washoils, for example, in amounts of up to about 10% by weight, andpreferably of from about 2.5 to 5% by weight--each relative to the oilphase.

However, especially suitable are also W/O emulsifiers having an improvedecological compatibility such as those described, more specifically, inthe older applications by one of the Applicants DE-A-40 03 028, DE-A-4024 658, DE-A-40 24 659, DE-A-40 24 892 and DE-A-41 02 908. In theseolder applications, there have been described, as W/O emulsifiers,selected emulsifiers based on ethers, on α-sulfofatty acid di-salts, onalkyl glycoside compounds, on surface-active ester sulfonate salts andon surface-active complex esters, respectively.

As to the continuous oil phase in the W/O invert fluid used in aparticular drilling operation as a main working agent, in accordancewith the teaching of the invention nearly any optional mixing ratiobetween mineral oil and O-functionalized wash oil may be employed. Theselection will have to consider the actual conditions of the exchange ofthe oil phases and, if applicable, the process modification as mentionedabove of a first washing step using a mineral oil-based wash oil andrequiring just a final exchange of the oil phase on the cuttings'surfaces against the biologically compatible O-functionalized wash oilsaccording to the invention. Even if the proportion of thisO-functionalized wash oils in the invert drilling fluid duringoperation, eventually, may even comprise more than 50% by volume of thecontinuous oil phase, it is nevertheless preferred according to theinvention--e.g. for economic reasons--to keep the proportion of theO-functionalized wash oils in the invert drilling fluid below 50% byvolume, or by weight, of the continuous oil phase in the drilling fluid.It is readily possible here to work with by far lower amounts of thecomparably more expensive ecologically compatible and biologicallyacceptable wash oils in the invert mud. The amount of esters, ethersand/or alcohols of the described types in the invert fluid, thus, may beless than 40% by weight, and preferably not more than about from 20 to30% by weight, of the oil phase, and in particular not more than aboutfrom 5 to 15% by weight, of this oil phase. Even so, it will have to beensured by means of the measures according to the invention that amineral oil-free or virtually mineral oil-free solid material can beprovided for the decontamination by a biological decomposition.

The composition of the invert drilling muds which are preferred to bepredominantly mineral oil-based, moreover, corresponds to theconventional procedural instructions. Here, reference may be made to theolder applications as intially set forth. Thus, there may be summarized,only by way of an excerpt, the following:

Invert drilling muds usually contain the finely dispersed aqueous phasein amounts of from about 5 to 50% by weight together with the continuousoil phase. In addition to the water contents, all of the additivesprovided for comparable types of fluids are to be taken intoconsideration. These additives may be water-soluble, oil-soluble and/orwater- or oil-dispersible, respectively.

Conventional additives, in addition to the emulsifiers, include, forexample, fluid-loss additives, soluble and/or insoluble materials tobuild-up structural viscosity, alkali reserve, agents for inhibiting anundesirable water exchange between drilled formations--e.g.water-swellable clays and/or salt layers--and the drilling fluid,wetting agents for an improved strike of the emulsified oil phase onsolid surfaces, e.g. for improving the lubricating effect, but also forimproving the oleophilic closure of exposed rock formations or rocksurfaces, biocides, for example for inhibiting bacterial onset andgrowth on the emulsions and the like. In detail, reference is here to bemade to pertinent prior art such as described, for example, in thetechnical literature as initially quoted; cf., more specifically, Grayand Darley, loc. cit., Chapter 11, "Drilling Fluid Components". Just byway of an excerpt, there may be quoted accordingly:

Finely dispersed additives for increasing the density of the fluid:Widely used is barium sulfate (baryte), but also calcium carbonate(calcite) or the mixed carbonate of calcium and magnesium (dolomite) areused.

Agents for a build-up of structural viscosity which simultaneously willact as fluid-loss additives: Here, bentonite or hydrophobized bentoniteare to be mentioned in the first place. Also the concomitant use oforganic polymer compounds of natural and/or synthetic origin may be ofconsiderable importance in this connection.

Additives inhibiting the undesirable water-exchange with, for example,clays: Here to be considered are the additives known from prior art foroil- and water-based drilling fluids. These include, more specifically,halides and/or carbonates of the alkali and/or alkaline earth metals,whereof the potassium salts, optionally in combination with lime, may beof particular importance. Here, more recent proposals suggest the use ofwater-soluble lower alcohols such as glycerol and/or propanediol.Reference may also be made, for example, to the relevant publications in"Petroleum Engineer International", September 1987, 32-40, and "WorldOil", November 1983, 93-97.

Alkali reserves: Here to be taken into consideration are inorganicand/or organic bases adjusted to match the total behavior of the fluid,and more particularly appropriate basic salts or hydroxides of alkaliand/or alkaline earth metals as well as organic bases. Lime is anespecially important representative of this class. Kind and amount ofthese basic components will have been mutually adjusted in a knownmanner.

Basically, the amounts of each of the auxiliary materials and additivesis within the conventional range and, thus, may be learnt from therelevant literature as quoted.

EXAMPLES

The following investigations were made with a sample of drillingcuttings obtained from a current drilling operation on a field in theNorth Sea. Type of the drilling fluid: W/O invert drilling fluid, basedon mineral oil. The mineral oil content of the cuttings contaminatedwith drilling fluid was 13.8% by weight.

One hundred grams of drilling cuttings contaminated with the drillingfluid are admixed with 100 g of a monocarboxylic acid ester oil(commercial product "PETROFREE") and the resulting product wasthoroughly mixed in a beaker at 40° C. for 5 minutes. Then the solidswere separated off by vacuum filtration over a glass nutsch filter. Theproduct yield of the wet cuttings was 97.7 g.

The cuttings thus isolated were then subjected to a Soxhlet extractionwith n-hexane for 8 hours. The resulting extract is carefullyconcentrated at room temperature.

Eight grams of the extract thus recovered were applied onto a silicagelcolumn {column: inner diameter 20 mm; effective length 35 mm; filledwith silicagel 60 from the company Merck, Darmstadt, Germany (particlesize distribution from 0.063 to 0.2 mm)}. The applied mineral oilextract was then eluated through the silica gel column with 250 ml ofn-hexane. The eluate was collected. In this procedure, the "PETROFREE"monocarboxylic acid ester and other polar components of the extractremained attached to the silica gel column.

From the eluate collected at the bottom of the silica gel column, then-hexane was removed by blowing an air stream over the surface thereofuntil the weight of the residue remains constant. The purified mineralresidue thus obtained was weighed and related to the initial amount ofthe total sample (100 g).

The mineral oil content thus determined of the cuttings washed with themonocarboxylic acid ester was 1.709 g, which amount corresponds to aresidual mineral oil content of the drill cuttings of 1.75% by weight.

We claim:
 1. The process of washing drill cuttings containing mineraloil prior to disposal of said drill cuttings comprising washing saiddrill cuttings with a wash fluid consisting essentially ofbiologically-degradable oils having a flash point of about 80° C.selected from the group consisting of oleophilic monocarboxylic acidesters, oleophilic polycarboxylic acid esters, corresponding ethers,alcohols, or partial ethers thereof, and oleophilic carbonic acidesters, whereby said mineral oil is removed by saidbiologically-degradable oils.
 2. A process as in claim 1 wherein saidbiologically-degradable oils are compatible for use as a mixturecomponent of the continuous oil phase of water-in-oil invert emulsiondrilling fluids.
 3. A process as in claim 1 wherein saidbiologically-degradable oils have pour points and setting points ofbelow 0° C., and are flowable and pumpable at a temperature of from 0°C. to 5° C. and have a Brookfield viscosity of not more than 60 mPa.s.4. A process as in claim 1 wherein said biologically-degradable oils arenon-toxicological.
 5. A process as in claim 1 wherein saidbiologically-degradable oils are esters selected from the groupconsisting of;a) esters of C₁ -C₅ monocarboxylic acids and mono- orpolyfunctional alcohols, wherein said monofunctional alcohols contain atleast 6 carbon atoms and said polyfunctional alcohols contain from 2 to6 carbon atoms in the molecule, b) esters of saturated C₆ -C₁₆monocarboxylic acids and mono- or polyfunctional alcohols as incomponent a), and c) esters of olefinically mono- or polyunsaturated C₁₆-C₂₄ monocarboxylic acids and monofunctional or straight-chain branchedalcohols.
 6. A process as in claim 1 wherein saidbiologically-degradable oils are ethers selected from the groupconsisting of ethers derived from straight-chain monofunctional alcoholshaving at least 6 carbon atoms, ethers of monofunctional alcohols having8 to 12 carbon atoms, and mixtures thereof.
 7. A process as in claim 1including adding said biologically-degradable oils and the mineral oilwashed from said drill cuttings to an oil-based drilling fluid.
 8. Aprocess as in claim 1 including conducting at least one prewashing stepprior to washing wherein a mineral oil-based washing oil is optionallyemployed in said at least one prewashing step.
 9. A process as in claim1 including conducting at least one prewashing step prior to washingwherein said at least one prewashing step is carried out with the use ofmineral oil fractions employed in formulating drilling fluids, andwherein said mineral oil fractions are recycled to a drilling fluidstream after use in said prewashing step.
 10. A process as in claim 1wherein the wash fluid is flowed co-currently or counter-currently withrespect to a flow of said drill cuttings at a temperature of up to about100° C.
 11. A process as in claim 1 wherein said wash fluid includes anemulsifier.
 12. A process as in claim 1 wherein said washing isconducted immediately after recovering said drill cuttings during adrilling operation.
 13. A process as in claim 12 wherein said washing isconducted on an oil drilling platform.